My cat used to absolutely vanish for hours; I would search every conceivable spot a cat could get into and not find her. Then she would nonchalantly wander into whatever room I was in without a care in the world. This happened even after I moved two different times. I finally concluded that she was just an extra-dimensional being that didn't want to spend all its time in my dimension.

Then one day I was looking for her under my bed and I noticed a slight sagging of the fabric on the underside of the box spring. I poked it and sure enough she gave a slightly annoyed meow. A small corner of the fabric had come loose years before and she had just been crawling up into the box spring the whole time.